1. Field of the Invention
The present invention relates to methods and apparatus for detection of air ingress into cryogen vessels. It is particularly related to the detection of air ingress into cryogen vessels used to cool superconducting magnets used in imaging systems such as magnetic resonance imaging, nuclear magnetic resonance imaging and nuclear magnetic spectroscopy. The invention, however, may be applied to the detection of air ingress into any cryogen vessel.
2. Description of the Prior Art
FIG. 1 shows a conventional arrangement of a cryostat including a cryogen vessel that, when seen in the sectional view of FIG. 1 has an outer wall 12 and an inner wall 12a. A cooled superconducting magnet 10 is provided within the cryogen vessel, itself retained within an outer vacuum chamber (OVC) that, when seen in the sectional view of FIG. 1, has an outer wall 14 and an inner wall 14a. One or more thermal radiation shields 16 and 16a are provided in the vacuum space between the cryogen vessel and the outer vacuum chamber. In some known arrangements, a refrigerator 17 is mounted in a refrigerator sock 15 located in a turret 18 provided for the purpose, toward the side of the cryostat. Alternatively, a refrigerator 17 may be located within access turret 19, which retains access neck (vent tube) 20 mounted at the top of the cryostat. The refrigerator 17 provides active refrigeration to cool cryogen gas within the cryogen vessel, in some arrangements by re-condensing it into a liquid. The refrigerator 17 may also serve to cool the radiation shields 16 and/or 16a. As illustrated in FIG. 1, the refrigerator 17 may be a two-stage refrigerator. A first cooling stage is thermally linked to the radiation shields 16 and 16a, and provides cooling to a first temperature, typically in the region of 80-100K. A second cooling stage provides cooling of the cryogen gas to a much lower temperature, typically in the region of 4-10K.
A negative electrical connection 21a is usually provided to the magnet 10 through the body of the cryostat. A positive electrical connection 21 is usually provided by a conductor passing through the vent tube 20.
For fixed current lead designs, a separate vent path (auxiliary vent) (not shown in FIG. 1) may be provided as a fail-safe vent in case of blockage of the vent tube 20.
The cryogen 22 is typically liquid helium at a temperature of about 4K, although other cryogens may be used such as liquid hydrogen, liquid neon or liquid nitrogen. At service intervals, it is necessary to remove the refrigerator 17, and to open the vent tube 20. There is a risk that air could enter the cryogen vessel when the refrigerator is removed, or when the vent tube 20 is opened.
If air enters the cryogen vessel, it will be frozen as a frost, near its ingress point. With higher-temperature cryogens, such as nitrogen, only the water contained in air may be frozen. In any case, a frost will be deposited around the air ingress point. This may block the access for the refrigerator, which will degrade the performance of the refrigerator, leading to a rise on temperature and pressure within the cryogen vessel, in turn leading to increased consumption of cryogen. The frost deposit may build up around the vent tube 20. The vent tube serves to allow boiled-off cryogen gas to escape from the cryogen vessel, and is particularly important in the case of a magnet quench. During a magnet quench, a superconductive magnet suddenly becomes resistive, and loses all of its stored energy to the cryogen. This results in very rapid boil-off of cryogen. If the vent tube is constricted, or even blocked, then dangerously high pressure may build up within the cryogen vessel.
Removal of a frost deposit from the inside of the cryogen vessel requires removing all of the cryogen and allowing the cryogen vessel and the magnet or other equipment within it to warm up—for example, to room temperature. This is a time consuming and costly process, as the removed cryogen will need to be replenished, and, in the case of a superconducting magnet, a shimming operation may need to be performed to correct any changes in magnetic field homogeneity which may have been brought about by the warming and re-cooling of the magnet. During this whole process, the apparatus cooled within the cryogen vessel, and the system of which it forms a part, is unusable. This may have consequential effects such as patients being unable to be imaged, and maladies remaining undiagnosed. It is therefore not practical to warm the cryogen vessels and their contents as a preventative service operation. However, by not performing such preventative measures, the danger of blockages and excessive cryogen pressures remains.